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  icon Rare
  icon Not so common
  icon Common
  icon Low morbidity
  icon Considerable morbidity
  icon Serious


  • Rickettsiae: small pathogens in family Ricket-tsiaceae (Table 26-1)
    • Rickettsia
    • Orientia
    • Coxiella
    • Ehrlichia
  • Gram-negative coccobacilli/short bacilli; obligate localization/persistence within eukaryotic cells
  • Transmitted to humans by arthropods: tick, mite, flea, louse; mammalian reservoirs; humans are incidental hosts
  • Infections characterized by:
    • Exposure to vectors or animal reservoirs, travel to or residence in endemic locations
    • Clinical findings: fever, exanthem or tâche noire (black spot or stain) (coin-like lesion with central eschar and red halo at site of vector-feeding bite site) (Table 26-1), vasculitis, hepatosplenomegaly
    • Thrombocytopenia, leukopenia, elevated aminotransferase, hyponatremia
  • Diagnosis: confirmed by paired serum samples after convalescence or demonstration of microbes
  • Dermatopathology: microbes multiply in endothelial cells of small blood vessels and produce vasculitis with necrosis and thrombosis; disseminated intravascular coagulation (DIC) and vascular occlusion may occur
  • Doxycycline is drug of choice

Table Graphic Jump Location
Table 26-1 Classification of Groups of Rickettsial Infections and Clinical Features

Rickettsiae can cause life-threatening infections. Order of decreasing case-fatality rate: R. rickettsii [Rocky Mountain spotted fever (RMSF)]; R. prowazekii (epidemic louse-borne typhus); Orientia tsutsugamushi (scrub typhus); R. conorii [Mediterranean spotted fever (MSF)]; R. typhi (endemic murine typhus); in rare cases, other spotted fever group organisms.


ICD-9:066.1 • ICD-10: A77

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Table Graphic Jump Location
Table 26-2 Classification of Tick-Borne Rickettsial Spotted Fevers

Rocky Mountain Spotted Fever


ICD-9: O82.0 • ICD-10: A77   Image not available. Image not available.


  • Zoonosis
  • Etiology: R. rickettsii
  • Vector: various ticks
  • Geography: occurs throughout Western Hemisphere (Americas)
  • Most severe of rickettsial infections
  • Classic triad: fever, rash, history of tick bite (not common)
  • Clinical findings: sudden onset of fever, severe headache, myalgia, characteristic acral exanthem spreading centripetally
  • Course: associated with significant morbidity and mortality rates
  • Synonyms: black measles, tick ...

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